The present invention relates to an optical movement information detector and electronic equipment having the same.
As a general rule, when a light source and an observer move relative to each other, light is subjected to frequency changes by the Doppler effect. The laser Doppler velocimeter (hereinafter, referred to as LDV) uses this effect to measure the moving velocity of an object to be measured by applying laser light onto the object and measuring Doppler frequency shifts in the scattered light derived from the object. This LDV, which was released by Yeh and Cummins in 1964 (Appl. Phys. Lett. 4-10 (1964) 176), is widely known and is in practical use today.
FIG. 12 shows an optical system diagram of a typical conventional LDV.
In FIG. 12, 101 denotes a laser diode (hereinafter, referred to as LD) as a semiconductor laser, 102 denotes a photodiode as a light detecting device (hereinafter, referred to as PD), 103 denotes a diffraction grating, 104 denotes a collimator lens (hereinafter, referred to as CL), 105 denotes a mirror, 106 denotes a condenser lens, 107 denotes a first luminous flux, or beam of positive first order diffracted light by the diffraction grating 103, 108 denotes a second beam of negative first order diffracted light by the diffraction grating 103, and 113 denotes an object to be measured.
In the optical system as constituted above, laser light emitted from the LD 101 is converted by the CL 104 into a parallel beam, and then is split into positive and negative first order diffracted lights at a diffraction angle of θ by the diffraction grating 103 to become the first beam 107 and the second beam 108. The first beam 107 and the second beam 108 are respectively reflected by the mirror 105 and are then made incident on the surface of the object 113 at an incident angle of θ to be overlapped each other again. The first beam 107 and the second beam 108 scattered by the object 113, which have been Doppler frequency-shifted, are slightly different from the LD 101 in oscillating frequency. As a result, the interferential waves of the first beam 107 and the second beam 108 scattered by the object 113 generate beat. This beat is termed beat signal. The moving velocity of the object 113 is obtained by heterodyne-detecting the beat frequency of the beat signal using the PD 102. Hereinafter, this typical conventional LDV will be described in further detail.
Here, when the direction in which the object 113 moves to the right, as shown in FIG. 15 is set as the normal direction, the first beam 107 is Doppler frequency-shifted by −fd and the second beam 108 is Doppler frequency-shifted by +fd, so that the apparent frequency of the first beam 107 becomes (f0−fd) and the apparent frequency of the second beam 108 becomes (f0+fd). Note that f0 represents the oscillating frequency of the LD 101. In this case, since an electric field of the light emitted from the LD 101 is represented as E0·cos (2πf0t), the first beam 107 is indicated by Equation (1) below and the second beam 108 by Equation (2) below:IA=EA·cos {2π(f0−fd)t+φA}  (1)IB=EB·cos {2π(f0+fd)t+φB}  (2)where f0 denotes a frequency of outgoing beam from the LD 101, E0 denotes an amplitude of the outgoing beam from the LD 101, EA denotes an amplitude of the first beam 107, EB denotes an amplitude of the second beam 108, φA denotes a phase of the first beam 107 and φB denotes a phase of the second beam 108.
Since the frequency of light is generally 100 THz (1014 Hz), it is impossible to measure the frequency information of Equation (1) and Equation (2) directly. Therefore, heterodyne detection is generally employed for direct measurement as mentioned above, and because f0>>fd is established, the interferential waves of Equation (1) and Equation (2) can be described by the following expression:
                              〈                                                                                    I                  A                                +                                  I                  B                                                                    2                    〉                =                                                            E                A                2                            +                              E                B                2                                      2                    +                                                    E                A                            ·                              E                B                            ·              cos                        ⁢                          {                                                2                  ⁢                                      π                    ⁡                                          (                                              2                        ⁢                                                  f                          d                                                                    )                                                        ⁢                  t                                -                                  (                                                            ϕ                      A                                        -                                          ϕ                      B                                                        )                                            }                                                          (        3        )            Note that < > in the left side of Equation (3) represents time average. Consequently, the PD 102 allows the frequencies of these interferential waves to be measured.
FIG. 13 shows a case in which the object 113 moves at a velocity of V, two beams are made incident on the object 113 at arbitrary angles of α and β respectively, and the observation point receives scattered light at an arbitrary angle of γ.
Frequency shift quantity due to the Doppler effect, which is obtained using the Lorentz transformation based on relativism in a precise sense, may be approximately obtained as follows when the moving velocity V is sufficiently smaller than velocity of light c. Relative velocities VA1 and VB1 of light from a light source A and a light source B and a moving object are expressed by the following equations:VA1=c−V sin αVB1=c+V sin β  (4)Also, apparent frequencies fA1 and fB1 of lights as seen from the object 113 are expressed by the following equations:
                                          f            A1                    =                                                    V                A1                            λ                        =                                          1                λ                            ·                              (                                  c                  -                                      V                    ⁢                                                                                  ⁢                    sin                    ⁢                                                                                  ⁢                    α                                                  )                                                    ⁢                                  ⁢                              f            B1                    =                                                    V                B1                            λ                        =                                          1                λ                            ·                              (                                  c                  +                                      V                    ⁢                                                                                  ⁢                    sin                    ⁢                                                                                  ⁢                    β                                                  )                                                                        (        5        )            Relative velocities VA2 and VB2 of scattered (reflected) lights relative to the object 113 are expressed by the following equations:VA2=c−V sin γVB2=c−V sin γ  (6)Consequently, frequencies fA2 and fB2 of the lights as seen from the observation point are expressed by the following equations:
                                          f            A2                    =                                                    c                                  V                  A2                                            ·                              f                A1                                      =                                          c                λ                            ·                                                1                  -                                                                                    V                        c                                            ·                      sin                                        ⁢                                                                                  ⁢                    α                                                                    1                  -                                                                                    V                        c                                            ·                      sin                                        ⁢                                                                                  ⁢                    γ                                                                                      ⁢                                  ⁢                              f            B2                    =                                                    c                                  V                  B2                                            ·                              f                B1                                      =                                          c                λ                            ·                                                1                  +                                                                                    V                        c                                            ·                      sin                                        ⁢                                                                                  ⁢                    β                                                                    1                  -                                                                                    V                        c                                            ·                      sin                                        ⁢                                                                                  ⁢                    γ                                                                                                          (        7        )            The difference between the frequency in Equation (7) and the frequency f0 (=c/λ) of incident light is a Doppler frequency shift quantity fd. Here, the beat frequency of the two beams measured at the observation point 2fd is expressed by the following equation using c>>V:
                                                                        2                ⁢                                  f                  d                                            =                                                                                    f                    B2                                    -                                      f                    A1                                                                                                                                          =                                                V                  λ                                ·                                  (                                                            sin                      ⁢                                                                                          ⁢                      α                                        +                                          sin                      ⁢                                                                                          ⁢                      β                                                        )                                                                                        (        8        )            It can be seen that 2fd is independent of a position of the observation point (angle: γ). In FIG. 12, in which α=β=θ is valid, the following expression is established based on Equation (8) according to the typical optical system of the LDV of FIG. 12:
                              2          ⁢                      f            d                          =                                                            2                ⁢                V                            λ                        ·            sin                    ⁢                                          ⁢          θ                                    (        9        )            Consequently, the moving velocity V of the object 113 is obtained by measuring frequency 2fd indicated in Equation (3) and performing calculation using Equation (9).
Equation (9) may be geometrically interpreted as follows: FIG. 14 is an enlarged view of an area in which the two beams in FIG. 12 (the first beam 107 and the second beam 108) overlap each other again. The two beams intersect at incident angles of θ respectively, and the broken lines in FIG. 14 show parts of the equal wave fronts of the beams. An interval between the broken lines shows the wavelength λ of light. The vertical heavy lines show the bright parts of interference fringes, and given that the interval between the vertical heavy lines is Δ, this Δ is obtained from Equation (10) below:
                    Δ        =                  λ                      2            ⁢            sin            ⁢                                                  ⁢            θ                                              (        10        )            
As shown in FIG. 14, when an object (shown as ●) passes perpendicularly to the interference fringes at a velocity of V, the frequency f is expressed by the following equation:
                    f        =                              V            Δ                    =                                                                                          2                    ⁢                    V                                    λ                                ·                sin                            ⁢                                                          ⁢              θ                        =                          2              ⁢                              f                d                                                                        (        11        )            This equation is equal to Equation (9).
The mentioned typical LDV can thus obtain the moving velocity V; however, the LDV cannot detect the moving direction of the object to be measured. In contrast, in JP 03-235060 A, detecting a moving direction is made possible by rotating the diffraction grating 103 in FIG. 12 at a velocity of Vg. As a result, when light is reflected by the diffraction grating 103, each of beams is subjected to the Doppler frequency shift in proportion to Vg. Accordingly, the beat frequency 2fd to be measured in the PD 102 is indicated by the following equation:
                              2          ⁢                      f            d                          =                                                            2                ⁢                V                            λ                        ·                          (                              V                +                                  V                  g                                            )                        ·            sin                    ⁢                                          ⁢          θ                                    (        12        )            Consequently, the moving direction is obtained since the magnitude of 2fd is determined according to the positive or negative sign of the moving velocity V relative to a given velocity of Vg. According to the abovementioned optical system, however, a rotating mechanism of the diffraction grating 103 is required with result that the device becomes larger in size and higher in cost. In addition, in the optical system, the rotational velocity of the diffraction grating 103, which needs to be precisely maintained, is difficult to maintain due to factors such as errors caused by eccentricity, vibration caused by rotation, etc. Thus, the optical system is difficult to employ for precise measurement.
A velocimeter which solves the above problems is disclosed in JP 04-204104 A. The velocimeter uses a frequency shifter to change the frequency of an incident beam, which allows detection of the moving direction of an object to be measured.
FIG. 15 shows a schematic diagram of an optical system of the velocimeter.
According to the velocimeter, light emitted from a laser source 1 become a parallel beam by a CL 104, and then are split into two beams by a beam splitter (hereinafter, referred to as BS) 109. The beams are reflected by a mirror 105 and are then frequency-shifted by f1 and f2, respectively, by an acousto-optic modulator (hereinafter, referred to as AOM) 110. The light is again collected on the surface of an object to be measured 113 by a diffraction grating 103 so as for the beat frequency of scattered light from the object to be measured 113 to be detected using a PD 102. The frequency 2fd to be detected here is expressed by the following equation:
                              2          ⁢                      f            d                          =                              (                                                                          f                  1                                -                                  f                  2                                                                    )                    +                                                                      2                  ⁢                  V                                λ                            ·              sin                        ⁢                                                  ⁢            θ                                              (        13        )            Since the sign (plus or minus) of V changes according to the moving direction of the object 113, the moving direction of the object 113 is detected by the magnitude relationship of 2fd relative to a given frequency shift quantity |f1−f2|.
Also in JP 08-15435 A, frequency is changed using an electro-optical device (hereinafter, referred to as EOM) 111 shown in FIG. 16 based on the principle similar to the principle employed in JP 04-204104 A. More specifically, light emitted from an LD 101, which is a laser source, becomes a parallel beam by a CL 104, and is then split into two beams, a first beam 107 and a second beam 108, by a diffraction grating 103. The first beam 107 and the second beam 108 enter corresponding EOMs 111. Here, bias is applied to the second beam 108 to shift its frequency by fR. The first beam 107 and the second beam 108 are reflected by a mirror 105, and then are collected on the surface of the object to be measured 113. The beat frequency of scattered light from the surface of the object 113 is detected using a PD 102. The frequency 2fd detected here is expressed by the following equation:
                              2          ⁢                      f            d                          =                              f            R                    +                                                                      2                  ⁢                  V                                λ                            ·              sin                        ⁢                                                  ⁢            θ                                              (        14        )            Consequently, similarly to Equation (13), the moving direction of the object 113 is detected by the magnitude relation of 2fd relative to a given frequency shift quantity fR since the sign of V changes according to the moving direction of the object.
However, an optical system where the moving direction of the object 113 is detected using frequency shifters such as the AOM 110 and the EOM 111, is disadvantageous in that the device is made larger in size since the optical system becomes more complex and facilities for driving the frequency shifters such as a power source are required. For example, voltage necessary for frequency modulation by the AOM 110 is of about tens of volts and voltage necessary for frequency modulation by the EOM 111 is of about 100 volts with the result that a large-sized power source is required.
Further, in order to detect a two-dimensional moving velocity (a velocity component in a direction parallel to an arrow and a velocity component in a direction vertical to the drawing sheet in FIG. 15 and FIG. 16) with use of the above-described optical system, two optical systems are necessary. More specifically, two optical systems have to be disposed in such a manner that the velocity component in one direction is detected by one optical system and the velocity component in a direction orthogonal to the one direction is detected by the other optical system. Further, since scattered light from each of beam spots formed for detecting the components in the two directions diffuses in a spherical form, the diffused light from one beam spot will act as noise to a light detecting device for detecting light from the other beam spot, which makes it necessary to provide a system for separating optical signals. This causes a problem that detecting the two-dimensional moving velocity would complicate the structure of the optical system.